Build failure with "Expected initializer before 'final'" on `ArenaOptions`

[Emily-TTG]

Appears to be a similar issue as #16867

What version of protobuf and what language are you using?
6.32.1 with Abseil 20250814.1 (Both are the latest packages available from Conan)

Language: C++

OS: Arch Linux, up to date as of issue posting

Compiler Version: Unclear -- Conan appears to be building with GCC 15.2.1

What did you do?
Build a Conan project with the following options:

self.requires("abseil/20250814.1", override=True)
self.requires("protobuf/6.32.1", override=True)

I need to force a later abseil version to resolve abseil/abseil-cpp@809e5de

What did you expect to see
A working Conan run

What did you see instead?
Many errors from the Protobuf build starting with:

In file included from /mnt/library/.conan2/p/b/protoe096a7cdda60b/b/src/src/google/protobuf/arenastring.h:19,
                 from /mnt/library/.conan2/p/b/protoe096a7cdda60b/b/src/src/google/protobuf/arenastring.cc:8:
/mnt/library/.conan2/p/b/protoe096a7cdda60b/b/src/src/google/protobuf/arena.h:110:48: error: expected initializer before ‘final’
  110 | struct ABSL_ATTRIBUTE_WARN_UNUSED ArenaOptions final {

Anything else we should know about your project / environment
N/A

[sbenzaquen]

Doesn't seem related to #16867. That issue was mixing __declspec(xx) and [[xx]] attributes. There is no attribute mixing here.

Can you verify what is the expansion for ABSL_ATTRIBUTE_WARN_UNUSED in your toolchain for those two versions of Abseil?

[sbenzaquen]

Given that compiler error, I think the macro is somehow undefined in the configuration that fails to build for you.
I checked Abseil 20250814.1 and it has the attribute as expected.
https://github.com/abseil/abseil-cpp/blob/20250814.1/absl/base/attributes.h#L1071